1. Field of the Invention
The present invention relates to an inverter circuit, a backlight assembly and a flat panel display with the backlight assembly.
2. Description of the Related Art
Recently, information processing devices have rapidly advanced to have various shapes and functions. Information processed by such information processing devices is in the form of an electrical signal. Therefore, users require a display device to visually recognize the information processed by the information processing devices.
An example of the display device is a flat panel display device such as a liquid crystal display (“LCD”). An LCD displays an image using liquid crystal. When compared to other display devices, an LCD is thin and lightweight and also has a low power consumption and a low driving voltage. Therefore, an LCD is widely used in various fields.
Such an LCD including a liquid crystal panel displaying an image thereon and a backlight assembly providing light to the liquid crystal panel is described in Japanese Patent Publication No. 2005-49747, for example.
FIG. 1 is a schematic circuit diagram of a conventional backlight assembly.
Referring to FIG. 1, the conventional backlight assembly includes eight cold cathode fluorescent lamps (“CCFLs”) 810 and a balance circuit 820. As a liquid crystal panel increases in size, the conventional backlight assembly requires a plurality of CCFLs to provide uniform brightness in the liquid crystal panel.
Sinusoidal voltages are applied from an external inverter 800 to the CCFLs 810, and thus sinusoidal currents flow through the CCFLs 810. If the CCFLs 810 are driven by sinusoidal voltages with the same polarity, the sinusoidal voltages with the same polarity cause an interference with a driving circuit of a liquid crystal panel by generating noise of interference patterns on the liquid crystal panel. To prevent the interference, the CCFLs 810 are divided into two groups as illustrated in FIG. 1, and the two groups are driven respectively by high sinusoidal voltages with opposite polarities. That is, the inverter 800 is configured to output both a high positive voltage is and a high negative voltage. The odd-numbered CCFLs 810 and even-numbered CCFLs 810, starting from the top, are driven by the high positive voltage and the high negative voltage, respectively.
The CCFLs 810 have a negative resistance and are connected in parallel to one another. Therefore, when a current starts to flow through a given one of the CCFLs 810, a resistance of the given CCFL decreases and thus a current easily flows through the given CCFL. This causes concentration of the current at the given CCFL. To prevent the current concentration, the balance circuit 820 is connected in series to the CCFLs 810, as illustrated in FIG. 1.
The balance circuit 820 includes four balance transformers. Each of the balance transformers includes: a primary coil 821 connected directly between neighboring CCFLs receiving the high positive voltage and the high negative voltage, respectively; and a respective secondary coil 822 installed adjacent to the primary coil 821. When a current flows through any one of the CCFLs 810, a current flows through the primary coil 821 and thus a current flows through the respective secondary coil 822. Since the respective secondary coils 822 are connected in series to one another, the current flowing through the secondary coil 822 causes a current to flow through the corresponding primary coil 821. As a result, currents flowing through the respective CCFLs 810 become equal to one another.
In this configuration, a balancing voltage of each balance transformer that is necessary to balance the CCFLs 810 is obtained by grounding one point of the series connected secondary coils 822 and detecting a voltage between the grounded point and a detection node 830 remote from the grounded point. In a normal state, the balancing voltage is in the range of, for example, 1 V to 2 V.
This balancing voltage varies with a distribution of resistances including the negative resistances of the CCFLs 810. Using this property, a short or open circuit due to a failure in the CCFLs 810 can be detected. That is, when a short circuit occurs due to a failure in the CCFLs 810, a voltage (e.g., 5˜6 V) higher than the balancing voltage in the normal state is detected at the detection node 830 as a result of the balancing operation of the balance transformers.
FIG. 2 illustrates an arrangement of CCFLs in the conventional backlight assembly of FIG. 1.
Referring to FIG. 2, the CCFLs 810 are arranged horizontally in a vertically-standing protection structure 920. The rear surface of the protection structure 920 is covered with a reflection plate 910, and the front surface of the protection structure 920 is covered with a diffusion plate 900. In the conventional backlight assembly, temperature increases upward due to heat by light emitted from the CCFLs 810, resulting in a temperature gradient.
The CCFLs 810 each have a temperature-dependent resistance. Therefore, due to the temperature gradient, the CCFLs 810 have a resistance that is lowered as the CCFLs 810 are further spaced apart from a lower portion of protection structure 920. In other words, the CCFLS 810 have an increase in voltage due to the lowered resistance based on the temperature gradient from bottom to top of the protection structure 920 (e.g., the temperature increases from bottom to top of the protection structure 920).
To eliminate the resistance difference between the CCFLs 810, the balance transformers operate to balance the CCFLs 810. Accordingly, a voltage of, for example, about 3V is induced at the detection node 830. However, when an increase in voltage is detected at the detection node 830 in the conventional backlight assembly, it is impossible to determine whether a resistance difference between the CCFLs 810 or a short circuit due to a failure in the CCFLs 810 has caused the voltage increase. Accordingly, it is difficult to accurately trouble-shoot a failure in the CCFLs 810.